P/2013 R3 (Catalina–PanSTARRS)

In 2018, an orbit analysis by Henry Hsieh and collaborators tentatively determined that one of the asteroid's fragments, P/2013 R3-B, may be related to the 290,000-year-old Mandragora asteroid family of C-type asteroids.[7]^: 10 However, they were unable to link the other fragment, P/2013 R3-A, to this same family, possibly due to orbital perturbations by the 9:4 Jupiter mean-motion resonance, non-gravitational acceleration by outgassing, or uncertainties in the orbit of P/2013 R3-A.[7]^: 8 The process by which the Mandragora family formed is unclear due to the unusually uniform size of most of its members; possible explanations include a cratering impact event or a cascading rotational fissioning of the parent body and followed by its fragments.[10]^: 125

Illustration depicting the process of P/2013 R3's rotational disintegration by the YORP effect

The activity of P/2013 R3 was caused by the catastrophic rotational breakup of its nucleus, which progressively disintegrated into slow-moving fragments and dusty debris over several months.[5]^: 1 The combined cross-sectional areas of P/2013 R3's fragments suggest the asteroid's nucleus originally had a diameter of 800 m (2,600 ft) before it broke up.[5]^: 9 It is the first asteroid discovered while in the process of disintegrating.[5]^: 1

Rotational breakup occurs when an asteroid rotates faster than the critical spin barrier period at 2.2 hours, below which centrifugal forces exceed the gravitational and interparticle van der Waals cohesive forces holding the asteroid together.[5]^: 15[11]^: 1 Material along the asteroid's equator, where centrifugal forces are strongest, is ejected outward beyond the asteroid's gravitational escape speed (~0.5 m/s) and then disaggregates into discrete fragments due to interparticle cohesion.[12]^: 2[11]^: 1, 14

The precursor of P/2013 R3 gradually spun up past the critical period due to a continuous yet subtle net torque on the asteroid's rotation, primarily caused by sunlight reflecting off the asteroid's irregular surface—a phenomenon known as the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect. The YORP effect can spin up sub-kilometer-sized asteroids like P/2013 R3 below the critical period over a short timescale within one million years.[3]^: 5 Compared to the 150-million-year average rate of disruptive impact events for sub-kilometer main-belt asteroids, rotational breakup by the YORP effect occurs much more frequently by two orders of magnitude.[5]^: 15[13] Several active asteroids in the main belt are suspected of undergoing or approaching rotational breakup, such as 331P/Gibbs (P/2012 F5), 311P/PanSTARRS (P/2013 P5), and 6478 Gault.[14]^: 17, 19

In addition to the YORP effect, uneven outgassing of sublimating water ice may be in part responsible for spinning up P/2013 R3's precursor. Although there is no direct spectroscopic evidence of water sublimation in P/2013 R3, it is implied by the prolonged dust ejection of its fragments, whose subsurface ices are presumably exposed after the precursor's breakup.[3]^: 4 Without the YORP effect, constant water sublimation rates greater than 1×10⁻³ kg/s (1 g/s) can spin up sub-kilometer asteroids below the critical period in less than one million years and can be spectroscopically undetectable if the sublimation rate is below the observational lower limit of 1 kg/s.[5]^: 15–16[3]^: 3

The rotation periods of P/2013 R3's precursor and fragments have not been photometrically measured due to heavy obscuration by surrounding debris.[14]^: 7, 12 Depending on the precursor's size and density, the low relative speeds of P/2013 R3's fragments suggest that P/2013 R3's precursor had a rotation period between 0.48−1.9 hours before it began breaking up.[12]^: 4[5]^: 15 This is well below the critical spin period, which implies that P/2013 R3's particles must have cohesion forces resisting centrifugal forces at a cohesive strength of 40–210 Pa, comparable to that of a pile of sand or talcum powder.[12]^: 4[14]^: 19[9] This cohesive strength is characteristic of weakly-bound rubble pile asteroids like Bennu and Ryugu, whose internal structures have been thoroughly fractured by repeated meteoroid impacts over billions of years.[14]^: 9 If P/2013 R3 was a rubble pile of this cohesive strength, it should predominantly consist of micrometer-sized rock grains.[12]^: 4 However, this low cohesive strength fails to explain P/2013 R3's fragmentation according to computer simulations by Yun Zhang and collaborators in 2018, who find that rubble pile asteroids of P/2013 R3's size should begin fragmenting at higher cohesive strengths of at least 800 Pa.[11]^: 14

P/2013 R3 continuously released dusty debris into space for the entire span of its disintegration, producing an enveloping dust cloud and tail that gave it its diffuse, comet-like appearance.[5]^: 6 The dust cloud has a spectrally neutral or gray color matching those of primitive, carbonaceous C-type asteroids.[3]^: 3, 5 Because the dust cloud spans a large area for reflecting sunlight, it accounts for much of P/2013 R3's apparent brightness. Observations in October 2013 showed that the dust cloud initially had a scattering cross-sectional area of at least 30 km² (12 sq mi) before reducing to 20 km² (7.7 sq mi) by February 2014. During this time span, the total apparent magnitude of P/2013 R3 faded from magnitude 18 to 20.[3]^: 3–4[5]^: 10

In addition to the enveloping dust cloud, the individual fragments of P/2013 R3 each possessed their own comas and tails, sourced from dust ejected by rotational breakup and potentially sublimating water ice.[5]^: 5, 12[14]^: 19 Based on the fading rate of P/2013 R3's fragments, their dust ejection speed is inferred to be at least 0.13 m/s (0.43 ft/s).[5]^: 12

Smaller dust particles were blown away by solar radiation pressure, leaving relatively large micrometer- to millimeter-sized particles behind.[3]^: 3[5]^: 6

Assuming that P/2013 R3's dust grains follow a similar size distribution to other fragmented asteroids and comets, the asteroid is estimated to have ejected a total of 2.3×10¹⁰ kg of dust over the course of its disintegration. This dust mass is equivalent to a 340 m (1,120 ft)-diameter solid sphere, for an assumed uniform density of 1.0 g/cm³.[5]^: 11

The asteroidal dust production rate contributes at least 4–20% to the zodiacal cloud.[5]^: 16

P/2013 R3-A, also known as component A, was the brightest and presumed main component of P/2013 R3.[5]^: 3[15] It was the southwestern and trailing component of the pair of nuclei initially seen at discovery.[1]

High-resolution observations by Hubble on 29 October 2013 revealed that component A was composed of four smaller fragments, named A1, A2, A3, and A4, that were embedded together in a 2,000 km (1,200 mi)-wide dust coma.[3]^: 4 Three of these fragments had just separated from each other a few days prior to these Hubble observations.[5]^: 3, 9 Fragment A1 was the brightest fragment of component A and it is thought to be the parent body of all other fragments, potentially including components B, C, and D, but excluding A3.[5]^: 9–10 Fragment A3 is instead thought to have split off from fragment A2 around the time when it was first seen by Hubble on 29 October 2013.[5]^: 9

By the time Hubble reobserved P/2013 R3 on 15 November 2013, fragment A3 had disappeared completely and fragment A1 had ejected two new fragments, named A5 and A6. Hubble observations on 13 December 2013 later found that fragment A2 had ejected another small fragment, named A7, which had separated sometime in between November and December at a relative speed of 0.46 m/s.[5]^: 6, 9 The six remaining fragments of component A faded over time, with fragment A4 disappearing sometime in between 13 December 2013 and 14 January 2014. The fading eventually progressed to the point that the fragments of component A became unidentifiable by 13 February 2014.[5]^: 6

Final Hubble image of P/2013 R3 and its fragments (labeled) on 13 February 2014

P/2013 R3-B, also known as component B,[lower-alpha 1] was the northeastern and leading component of the pair of nuclei initially seen at discovery.[1] It was 23% as bright as component A and it was separated 3.4 arcseconds apart (projected distance of 3,100 km or 1,900 mi) when it was observed by the Gran Telescopio Canarias on 11 and 12 October 2013.[15]

Observations by Hubble on 29 October 2013 showed that component B was initially a single object possessing its own dusty coma and tail. On 15 November 2013, component B began to split into two fragments, B1 and B2, that eventually moved far enough apart to be resolved by Hubble on 13 December 2013.[5]^: 12 The fainter fragment, B2, separated from B1 at a relative speed of 0.32 m/s and faded steadily with constant absolute magnitude.[5]^{: 3, 9, 12} On the other hand, the brighter fragment B1 had its absolute magnitude brighten by more than a magnitude between 13 December 2013 and 14 January 2014, before ejecting another small fragment, B3, that was seen by Hubble on 13 February 2014. This sudden brightening of B1 may be associated with the beginning of B3's separation, based on similar behavior seen in comets before fragmenting.[5]^: 12

Schematic diagram illustrating the sizes and relationships between the fragments of P/2013 R3

A faint fragment, named component C,[lower-alpha 1] was first seen southwest of component A in Keck Telescope observations on 1 and 2 October 2013.[3]^: 2 It was detected again in Gran Telescopio Canarias observations on 11 and 12 October 2013 and 6.5-m Magellan Telescope observations on 28 and 29 October 2013.[15][5]^: 4 Component C was 5–10% as bright as component A and it was separated 2.4 arcseconds apart (projected distance of 2,500 km or 1,600 mi) at the time of the Gran Telescopio Canarias observations.[15]

Observations by Hubble on 29 October 2013 revealed that component C had already split into two fragments, C1 and C2. With an area-projected diameter of 120 m (390 ft), fragment C1 was the smallest of the two fragments and it had promptly faded to invisibility by the time Hubble reobserved P/2013 R3 on 15 November 2013.[5]^: 3 The surviving fragment, C2, faded at a slower rate and did not experience any further fragmentation over the course of Hubble observations through 2013–2014.[5]^: 4 Based on its last observed brightness on 13 February 2014, fragment C2's area-projected diameter was around 160 m (520 ft).[5]^: 10

An additional faint fragment, named component D,[lower-alpha 1] was discovered trailing far behind the main components' tail in Keck observations from 1 and 2 October 2013.[3]^: 1[5]^: 4 Component D was detected again in Gran Telescopio Canarias observations on 11 and 12 October 2013. The fragment was 0.5% as bright as component A and it was widely separated 36 arcseconds apart (projected distance of 32,700 km or 20,300 mi) at the time of the Gran Telescopio Canarias observations.[15] It was possibly last seen in Magellan Telescope observations on 29 October 2013, although it may have become too faint for confident identification.[3]^: 2[5]^: 4

For an assumed geometric albedo of 0.05 with minimal dust contamination, component D is less than 200 m (660 ft) in diameter.[5]^: 3 The separation time of component D is uncertain due to the lack of relative motion to other components during its short observational coverage. Keck observations from 1 and 2 October 2013 place an upper limit separation speed of ≤6 m/s, corresponding to a separation date sometime earlier than 30 July 2013.[5]^: 5 Component D's latest possible separation dates overlap with the separation date range for P/2013 R3's dust cloud, suggesting that these two events may be related.[5]^: 13